Photosensitive resins that utilize photopolymerization of polymerizable unsaturated monomers have been used in various application areas including photopolymer printing plates, ultraviolet ray curable inks, photocurable paints, photoresists and the like.
However, in the heretofore known photosensitive resins, the photopolymerizable unsaturated monomers generally used are acrylic or methacrylic esters. These monomers tend to give, when photopolymerized, vitreous polymers at normal temperatures and therefore the resinous compositions thus produced are hard, brittle cured products. These brittle resinous products tend to crack or peel off of the said layer from the supporting substrate when the photopolymer printing plate is subjected to a bending operation. This is especially true in the case of photosensitive resin compositions whose image formation is carried out by development with water, because a water soluble polymer is customarily used as polymeric binder in such composition and the compound itself is of a hard nature.
In addition, water-developable photopolymer printing plates which are useful in the field of flexographic printing that requires flexible and rubber like resilient plates, are unknown.
As examples of the abovesaid type of photosensitive resin composition, there have been employed the combination of a water soluble polymer (e.g. polyvinyl alcohol, a polyacrylamide, a partially saponified polyvinyl acetate, a methyl cellulose, a hydroxy propyl methyl cellulose and the like), a water soluble (meth)acrylic ester (e.g. 2-hydroxyethyl methacrylate, a 2-hydroxypropyl methacrylate, a polyethyleneglycol dimethacrylate and the like, and a photopolymerization initiator (see Japanese Patent Publication No. 27561/77). In this type of composition, there are such fatal drawbacks that the photopolymerized product of 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate or the like is of a vitreous nature at normal temperature, and when the water soluble polymer selected is, for example, polyvinyl alcohol, cracks occur in the formed plate, especially at the drying season in winter and such plate cannot be used in the field of flexo printing because it lacks resilience and flexibility.
Furthermore, in the abovesaid (meth)acrylic monomer system, there are additional problems such that since the photopolymerization speed is low and polymerization inhibiting effect of oxygen is high, it is difficult to produce a good quality image, unless all of the oxygen contained in the layer are replaced by carbon dioxide gas or consumed by means of bump-exposure without a negative film before image forming exposure.
For the purpose of overcoming the aforesaid drawbacks possessed by the (meth)acrylic monomer system there has been proposed a combination of a condensation oligomer of alkylol or alkylated alkylol derivative of urea or thiourea with N-alkylol(meth)acrylamide, and a water soluble polymer (e.g. polyvinyl alcohol), (see U.S. Pat. No. 4,209,581).
This condensation oligomer is satisfactorily compatible with a water soluble polymer, has an improved photopolymerization speed and can supplement the aforesaid drawbacks possessed by a (meth)acrylic monomer system. However, the photocured product is very hard, and there still remains the question of fragility of the formed plate which makes it unsuitable in the flexo printing field.
Moreover, in a condensation polymerization reaction, formaldehyde is always by-produced and therefore, unless the formed formaldehyde is completely removed out of the reaction product by purification, the condensation product cannot be used in admixture with a water soluble polymer as polyvinyl alcohol, since polyvinyl alcohol reacts with formaldehyde.
The inventors, having made attempts to discover a material which is compatible with a water soluble polymer, and which can relieve the drawbacks possessed by heretofore used photopolymerizable unsaturated monomers, such drawbacks being the fragility of the formed plate due to the hardness and lower photocuring speed and which improved product can give sufficient flexibility and rubber like resilience so as to be used in the flexo printing field. The inventors have discovered a product which can overcome these difficulties by mixing a particular weight ratio of urea, thiourea or an alkyl derivative thereof and an N-alkylol acrylamide or an N-alkylol methacrylamide and on the basis of this finding have completed the invention.